I felt like you after taking a 3+ year hiatus from cycling. I bought a new bike and wanted to ride like I used to. After 2 weeks of slowly trying to get my foundation back, I still didn't feel like I was improving and I definitely didn't have power I used to. You know what I did? I took two weeks off the bike. During that time, I did some light weight-training (squats, lunges). BTW, you can do these without free weights. I also started working on my neglected core muscles and walking again. Then I eased back on the bike like the first 2 weeks. The improvements were immediately obvious, which I think mainly came from the core muscle work and giving my cycling muscles a break to FULLY recover.
My climbing still sucked after the second 2 weeks on the bike, so I started throwing in climbing days. I will warm up for about 30 minutes and then find some moderate hills to tackle. It's not hilly where I live (coastal plain) so I have to climb the same ones over and over, recovering/spinning for a 5-10 minutes when I get tired. Effect: climbing has improved.
I have just started working on my "power". A few days ago I went out and warmed up for about 30 minutes. Then I found a quiet, flat stretch to try my hand at some intervals. I could only hold a very hard effort for MAYBE 10-15 seconds. Kind of discouraging, but I would let myself fully recover and did a total of 3. Then I cooled down for 10-15 minutes and headed inside. Two days later (after not riding at all), I went on a "bonk training" ride this Saturday morning. I kept it easy as prescribed. After about an hour on the bike I was feeling pretty good so I decided to kick up the speed a little bit. I found myself EASILY maintaining 20 MPH on the flats. This kind of effort would immediately drain me a few weeks before.
This is what I have learned since getting back on the bike 2 months ago. As much as I hate it, it is going to take a little time to back into riding shape again. It's very easy to overtrain, especially when you are in a "hurry" to improve. Recovery seems to be just as important as the training. Keep taking the protein after your rides. I have noticed it helps how sore I feel the days after riding. I pretty much always drink Cytomax and some whey protein after every ride. I also rehydrate very well after especially hot rides.
Don't be afraid to take days off or do easy recovery rides. Also try to mix up your training.